Army Act 1914
Pt ii
Section 81
If a recruit within three months after the date of his attestation pays for the use of His Majesty a sum not exceeding ten pounds, he shall be discharged with all convenient speed, unless he claims such discharge during a period when soldiers in army service who otherwise would be transferred to the reserve are required by a proclamation of His Majesty in pursuance of this Act to continue in army service, in which case he may be retained in His Majesty's service during that period, and at the termination thereof, if he so require it, on the payment of that sum, be discharged.
So there you are...A proclamation was made at the start of the war, ergo he would not have been able to get out until the termination of that proclamation...ie the end of hostilities!!